Nanostructures with discrete domains of different materials are of great interest for the integration of multiple functionalities. While continuously being attracted by the classical hybrid nanomaterials, for example, core-shell, alloy, or some other bimetallic heterostructures, research interest has also been allured into the development of semiconductor-metal nanocomposites (also called nanohybrids) made up of different classes of materials with coherent interfaces. This type of nanostructure combines materials with distinct physical and chemical properties to yield a unique hybrid nanosystem having multifunctional capabilities and tuned or enhanced useful properties, which may not be attainable in individual materials.
The preparation of semiconductor-metal nanocomposites dates back 30 years or more. Early investigations were focused on the deposition/doping of different metals (Au, Ag, or Pt) on or in TiO2 powders for photocatalysis application. In these structures, the metal domain induces the charge equilibrium in photoexcited TiO2 nanocrystals, which has direct influence in dictating the energetics of the nanocomposites by shifting the Fermi level to more negative potentials. The Fermi level shift is indicative of improved charge separation in TiO2-metal systems and demonstrates its usefulness for enhancing the efficiency of photocatalytic reactions.
In 2004, synthesis of semiconductor-metal nanocomposites was demonstrated using a solution method to form nanohybrids by the selective growth of gold tips on the apexes of hexagonal-phase CdSe nanorods at room temperature. The new nanostructures display modified optical properties caused by the strong coupling between the gold and semiconductor sections. The gold tips show increased conductivity as well as selective chemical affinity for forming self-assembled chains of rods. The architecture of these nanostructures is qualitatively similar to bi-functional molecules such as dithiols that provide two-sided chemical connectivity for self-assembly and for electrical devices and can solve the difficult problem of contacting colloidal nanorods and tetrapods to the external world. Later the synthesis of asymmetric semiconductor-metal heterostructures was effected in which gold was grown on one side of CdSe nanocrystal rods and dots. Theoretical modelling and experimental analysis show that the one-side nanocomposites are transformed from the two-sided architectures, through a repining process. Subsequently, various approaches have been developed for the synthesis of semiconductor-metal nanocomposites by anisotropic growth of metals on semiconductors through reduction, physical deposition, or photochemistry. Examples include ZnO—Ag, CdS—Au, InAs—Au, TiO2—Co, PbS—Au, Ag2S—Au, and semiconductor-Pt systems.
Most recently, the present inventors contributed a general protocol to transfer the transition metal ions from water to an organic medium using an ethanol mediated method, which has been extended to synthesize a wide variety of heterogeneous semiconductor-noble metal nanocomposites (J. Yang, J. Y. Ying, Nat. Mater. 8, 683 (2009)). In other work, the inventors synthesized three different types of semiconductor-Au nanocomposites (CdS—Au. CdSe—Au and PbS—Au) via solution methods and tested their catalytic activities in the three-component coupling reaction of the model substrates benzaldehyde, piperidine and phenylacetylene in water. It was found that through the electronic coupling between semiconductor and Au domains, PbS—Au nanocomposites provided the highest activity, giving the desired propargylic amine product in isolated yield up to 95%.